Abstract:

A nitride semiconductor light-emitting device wherein a substrate or
nitride semiconductor layer has a defect concentration region and a low
defect density region other than the defect concentration region. A
portion including the defect concentration region of the nitride
semiconductor layer or substrate has a trench region deeper than the low
defect density region. Thus by digging the trench in the defect
concentration region, the growth detection is uniformized, and the
surface planarity is improved. The uniformity of the characteristic in
the wafer surface leads to improvement of the yield.

Claims:

1.-7. (canceled)

8. A nitride semiconductor light-emitting device comprising a substrate
and a nitride semiconductor layer laid on a top of the substrate,wherein
the substrate or the nitride semiconductor layer has a
defect-concentrated region and a low-defect region corresponding to a
region other than the defect-concentrated region,wherein the
defect-concentrated region or the low-defect region has a depression,
andwherein there is provided an engraved region obtained by engraving a
portion including the depression.

9. The nitride semiconductor light-emitting device according to claim 8,
wherein the depression measures 0.5 μm or more in depth and 1 μm or
more in width.

10. The nitride semiconductor light-emitting device according to claim 8,
wherein an engraving depth of the engraved region is 0.5 μm or more
but 50 μm or less.

11. The nitride semiconductor light-emitting device according to claim 8,
wherein a distance from an edge of the engraved region to an edge of the
defect-concentrated region is 5 μm or more.

12. The nitride semiconductor light-emitting device according to claim
8,wherein the nitride semiconductor layer has a ridge portion serving as
a laser light waveguide region, andwherein the ridge portion is so formed
as to be located 5 μm or more away from an edge of the engraved
region.

13.-18. (canceled)

Description:

[0002]There have been fabricated prototypes of semiconductor laser devices
that oscillate in a region ranging from ultraviolet to visible light by
the use of a nitride semiconductor material as exemplified by GaN, AlN,
InN, and composite crystals thereof. For such purposes, GaN substrates
are typically used, and therefore they have been intensively researched
by a host of research-and-development institutions. At the moment,
however, no semiconductor laser devices offer satisfactorily long useful
lives, and accordingly what is most expected in them is longer useful
lives. It is known that the useful life of a semiconductor laser device
strongly depends on the density of defects (in the present specification,
defects refer to, for example, vacancies, interstitial atoms, and
dislocations in a crystal) that are present in a GaN substrate from the
beginning. The problem here is that substrates with low defect density,
however effective they may be believed to be in achieving longer useful
lives, are difficult to obtain, and therefore researches have been
eagerly done to achieve as much reduction in defect density as possible.

[0003]For example, Non-Patent Reference 1 reports fabricating a GaN
substrate by the following procedure. First, on a sapphire substrate, a
2.0 μm thick primer GaN layer is grown by MOCVD (metalorganic chemical
vapor deposition). Then, on top of this, a 0.1 μm thick SiO2 mask
pattern having regular stripe-shaped openings is formed. Then, further on
top, a 20 μm thick GaN layer is formed again by MOCVD. Now, a wafer is
obtained. This technology is called ELOG (epitaxially lateral overgrown),
which exploits lateral growth to reduce defects.

[0004]Further on top, a 200 μm thick GaN layer is formed by HVPE
(hydride vapor phase epitaxy), and then the sapphire substrate serving as
a primer layer is removed. In this way, a 150 μm thick GaN substrate
is produced. Next, the surface of the obtained GaN substrate is ground to
be flat. The thus obtained substrate includes, within a substrate
surface, a defect-concentrated region and a low-defect region, and, in
general, it is classified into a defect-concentrated region including
many defects in a part of SiO2 and a low-defect region being all the
remaining part of SiO2.

[0005]The problems here is, however, that the characteristics of a
semiconductor laser device fabricated by growing a nitride semiconductor
layer, by a growing process such as MOCVD, on a substrate including a
defect-concentrated region and low-defect region vary, resulting in a
remarkably low yield rate.

[0006]As a result of an intensive research on why the characteristics of a
semiconductor laser device fabricated by growing a nitride semiconductor
layer, by a growing process such as MOCVD, on a substrate including a
defect-concentrated region and low-defect region vary, resulting in a
remarkably low yield rate, the applicant of the present invention has
found out that this is because poor flatness of the film surface results
in poor surface morphology. Specifically, when a nitride semiconductor
layer (particularly, an InGaN layer used as an active layer) is grown on
an irregular surface of the film, the thickness and composition of the
layer vary depending on the surface irregularities of the film, and thus
greatly deviate from the set values. Furthermore, the applicant has found
out that the poor surface morphology greatly depends on the shape of the
defect-concentrated region in the nitride semiconductor layer. That is,
the applicant has found out that the growth direction and mode of a thin
film strongly depends on the shape of the defect-concentrated region, and
therefore the irregularly-shaped defect-concentrated region degrades the
flatness of the film surface, leading to poor surface morphology. Growing
a thin film such as an active layer on such an irregular surface causes
the device characteristics to vary.

[0007]These results are obtained in experiments conducted in the following
manner. First, a case where a nitride semiconductor layer is grown on a
substrate including a defect-concentrated region and a low-defect region
will be described. FIG. 16(a) is a sectional view of a conventional
semiconductor laser device, and FIG. 16(b) is a top view of FIG. 16A.
Reference numeral 10 represents a substrate including a
defect-concentrated region and a low-defect region, reference numeral 11
represents a defect-concentrated region, reference numeral 12 represents
a low-defect region, reference numeral 13 represents a nitride
semiconductor layer, and reference numeral 13a represents a surface of
the nitride semiconductor layer.

[0008]If a nitride semiconductor layer is grown directly on the substrate
10 (i.e., without performing any preliminary treatment for the substrate,
etc.), the growth rate of the defect-concentrated region is greatly
different from that of the low-defect region, because the
defect-concentrated region has lower crystallinity than the low-defect
region and may have a growth surface that does not appear in the
low-defect region. As a result, the defect-concentrated region grows at a
lower growth rate than the low-defect region, and thus growth hardly
occurs in the defect-concentrated region.

[0009]FIG. 17(a) is a top view showing how a nitride semiconductor layer
having defect-concentrated regions in the shape of lines grows, and FIG.
17(b) is a top view showing how a nitride semiconductor layer having
defect-concentrated regions in the shape of dots grows. In either case,
since growth hardly occurs in the defect-concentrated regions, growth is
started at the defect-concentrated region x and proceeds in the direction
indicated by arrow A, and growth is started at the defect-concentrated
region y and proceeds in the direction indicated by arrow B. When growth
occurs in two different directions in this way, the layer thickness in a
growth meet portion becomes different from that elsewhere, leading to
poor surface flatness.

[0010]FIG. 18 shows measurements of the roughness as measured in the
direction [11-20] perpendicular to the line-shaped defect-concentrated
region and in the direction [1-100] parallel thereto. The measurements
were made by using the "DEKTAK3ST" model manufactured by A SUBSIDIARY OF
VEECO INSTRUMENTS INC. The measurement was conducted under the following
conditions: measurement length: 600 μm; measurement time: 3s; probe
pressure: 30 mg; and horizontal resolution: 1 μm/sample. The level
difference between the highest and lowest parts, within the 600 μm
wide region in which the measurement was taken, was found to be 200 nm.
Here, the large grooves in the defect-concentrated regions are not
considered.

[0011]In addition, the growth meet portion was found to be a non-luminous
region. Thus, it can be said that the difference in thickness between the
layers within a wafer surface causes the device characteristics to vary.

DISCLOSURE OF THE INVENTION

[0012]In view of the conventionally encountered problems mentioned above,
it is an object of the present invention to provide a nitride
semiconductor light-emitting device that offers uniform characteristics
within a wafer surface and improves the yield rate.

[0013]To achieve the above object, according to one aspect of the present
invention, in a nitride semiconductor light-emitting device, a substrate
or a nitride semiconductor layer has a defect-concentrated region and a
low-defect region corresponding to a region other than the
defect-concentrated region, and, in a portion thereof including the
defect-concentrated region, an engraved region engraved so as to be
located lower than the low-defect region.

[0014]In this way, by engraving the defect-concentrated region, the growth
direction is made uniform and the surface flatness is improved, offering
uniform characteristics within a wafer surface. This makes it possible to
improve the yield rate.

[0015]In this nitride semiconductor light-emitting device, the
defect-concentrated region has the shape of a line or a dot, and the
engraved region has the shape of a line. Preferably, the engraving depth
of the engraved region is 0.5 μm or more but 50 μm or less.
Advisably, it is preferable that the distance from an edge of the
engraved region to an edge of the defect-concentrated region is 5 μm
or more. Moreover, it is preferable that the nitride semiconductor layer
has a ridge portion serving as a laser light waveguide region, and the
ridge portion is so formed as to be located 5 μm or more away from an
edge of the engraved region.

[0016]Moreover, according to another aspect of the present invention, in a
nitride semiconductor light-emitting device, a substrate or a nitride
semiconductor layer has a defect-concentrated region and a low-defect
region corresponding to a region other than the defect-concentrated
region, the defect-concentrated region or the low-defect region has a
depression, and there is provided an engraved region obtained by
engraving a portion including the depression.

[0017]In this way, when there is a depression, a portion including the
depression is engraved. This makes it possible to achieve uniform growth
and improve the surface flatness.

[0018]In this nitride semiconductor light-emitting device, it is
preferable that the depression measures 0.5 μm or more in depth and 1
μm or more in width. Advisably, it is preferable that the engraving
depth of the engraved region is 0.5 μm or more but 50 μm or less.
Moreover, it is preferable that the distance from an edge of the engraved
region to an edge of the defect-concentrated region is 5 μm or more.
Furthermore, it is preferable that the nitride semiconductor layer has a
ridge portion serving as a laser light waveguide region, and the ridge
portion is so formed as to be located 5 μm or more away from an edge
of the engraved region.

[0019]Moreover, according to still another aspect of the present
invention, in a nitride semiconductor light-emitting device, a substrate
or a nitride semiconductor layer has a defect-concentrated region and a
low-defect region corresponding to a region other than the
defect-concentrated region, the nitride semiconductor layer has a ridge
portion serving as a laser light waveguide region, and there is provided,
between the ridge portion and the defect-concentrated region, an engraved
region engraved so as to be located lower than the low-defect region.

[0020]In this way, the engraved region does not necessarily have to be
provided in the defect-concentrated region, but may be provided between
the ridge portion and the defect-concentrated region to achieve improved
surface flatness.

[0021]In this nitride semiconductor light-emitting device, the engraved
region has the shape of a line. Preferably, the engraving depth of the
engraved region is 0.5 μm or more but 50 μm or less. Advisably, it
is preferable that the distance from an edge of the engraved region to an
edge of the defect-concentrated region is 5 μm or more. Moreover, it
is preferable that the ridge portion is so formed as to be located 5
μm or more away from an edge of the engraved region. Furthermore, it
is preferable that the width of the engraved region is 3 μm or more
but 150 μm or less.

[0022]As described above, according to the present invention, when a
substrate or a nitride semiconductor layer has a defect-concentrated
region and a low-defect region corresponding to a region other than the
defect-concentrated region, the nitride semiconductor layer or the
substrate is provided, in a predetermined portion thereof, with an
engraved region engraved so as to be located lower than the low-defect
region. In this way, the growth direction is made uniform and the surface
flatness is improved, offering uniform characteristics within a wafer
surface. This makes it possible to improve the yield rate.

[0023]Moreover, by providing an engraved region, it is possible to release
the strains present within the nitride semiconductor layer and thus
suppress development of cracks.

[0024]It is to be noted that, in the present specification, a negative
index indicating a crystal plane or direction is represented by its
absolute value headed with a negative symbol "-" instead of overscoring
the figure as conventionally practiced in crystallography, because it is
impossible to do so herein.

[0025]Used as a substrate in the present invention may be a GaN substrate
in a freestanding state by removing a primer therefrom, as used in the
conventional example described earlier, or a GaN substrate just as it is
without removing a sapphire primer layer. That is, the following
description deals with examples that use a substrate having a
defect-concentrated region and a low-defect region on a surface thereof
on which a thin film of a nitride semiconductor laser has not yet been
grown by MOCVD.

BRIEF DESCRIPTION OF DRAWINGS

[0026]FIG. 1(a) is a sectional view of a nitride semiconductor laser
device of a first embodiment, and FIG. 1(b) is a top view of FIG. 1(a);

[0027]FIG. 2 is a sectional view showing the layer structure of a nitride
semiconductor layer;

[0028]FIG. 3(a) is an enlarged top view showing an example of a
defect-concentrated region, FIG. 3(b) is an enlarged top view showing an
example of a defect-concentrated region, and FIG. 3(c) is an enlarged top
view showing an example of a defect-concentrated region;

[0029]FIG. 4(a) is a top view of a substrate of the first embodiment, and
FIG. 4(b) is a sectional view of FIG. 4(a);

[0030]FIG. 5(a) is a diagram showing the surface flatness as measured in
the direction [11-20], and FIG. 5(b) is a diagram showing the surface
flatness as measured in the direction [1-100];

[0031]FIG. 6(a) is a top view showing how a nitride semiconductor layer
having defect-concentrated regions in the shape of lines grows, and FIG.
6(b) is a top view showing how a nitride semiconductor layer having
defect-concentrated regions in the shape of dots grows;

[0032]FIG. 7 is a diagram showing the relationship between the depth X of
an engraved region and the yield rate;

[0033]FIG. 8(a) is a top view of a substrate having defect-concentrated
regions in the shape of lines, and FIG. 8(b) is a top view of a substrate
having defect-concentrated regions in the shape of dots;

[0034]FIG. 9 is a diagram showing the relationship between the distance Y
and the yield rate;

[0035]FIG. 10 is a top view of a substrate of a second embodiment having
defect-concentrated regions in the shape of dots;

[0036]FIG. 11(a) is a top view of a substrate of a third embodiment, and
FIG. 11(b) is a sectional view of FIG. 11(a);

[0037]FIG. 12(a) is a top view of a substrate of a fourth embodiment, and
FIG. 12(b) is a sectional view of FIG. 12(a);

[0038]FIG. 13(a) is a top view showing how a nitride semiconductor layer
having no engraved region grows, and FIG. 13(b) is a top view showing how
a nitride semiconductor layer having an engraved region grows;

[0039]FIG. 14(a) is a top view of a substrate of a fifth embodiment, and
FIG. 14(b) is a sectional view of FIG. 14(a);

[0040]FIG. 15(a) is a sectional view of a nitride semiconductor laser
device of the fifth embodiment, and FIG. 15(b) is a top view of FIG.
15(a);

[0041]FIG. 16(a) is a sectional view of a conventional semiconductor laser
device, and FIG. 16(b) is a top view of FIG. 16(a);

[0042]FIG. 17(a) is a top view showing how a conventional nitride
semiconductor layer having defect-concentrated regions in the shape of
lines grows, and FIG. 17(b) is a top view showing how a conventional
nitride semiconductor layer having defect-concentrated regions in the
shape of dots grows; and

[0043]FIG. 18(a) is a diagram showing the surface flatness of a
conventional nitride semiconductor laser device as measured in the
direction [11-20], and FIG. 18(b) is a diagram showing the surface
flatness of a conventional nitride semiconductor laser device as measured
in the direction [1-100].

BEST MODE FOR CARRYING OUT THE INVENTION

[0044]Hereinafter, embodiments of the present invention will be described
with reference to the accompanying drawings.

First Embodiment

[0045]FIG. 1(a) is a sectional view of a nitride semiconductor laser
device, and FIG. 1(b) is a top view of FIG. 1(a). An n-type GaN substrate
10 includes, as a part thereof, a defect-concentrated region 11, all the
remaining part thereof being a low-defect region 12.

[0046]Note that, in the present specification, a defect-concentrated
region denotes a region where, as a result of subjecting a substrate or a
nitride semiconductor layer fabricated on a substrate to etching by
dipping it in a mixed acid liquid, namely a mixture of sulfuric acid and
phosphoric acid, heated to 250° C., many etch pits are observed,
attesting to concentration of defects (or dislocations, for example)
therein. On the other hand, a low-defect region denotes a region with
EPDs (etch pit densities) of the order of 104 to 105/cm2.
The defect-concentrated region has three or more orders of magnitude
greater EPDs. The measurement of the EPD can be made possible by the use
of gas-phase etching such as RIE (reactive ion etching). Alternatively,
suspension of growth in a MOCVD furnace followed by exposure to a high
temperature (about 1,000° C.) also makes the measurement of the
EPD possible. The measurement itself can be achieved by the use of an AFM
(atomic force microscope), CL (cathode luminescence), microscopic PL
(photo luminescence), or the like.

[0047]On the substrate 10, a nitride semiconductor layer 13 (an
epitaxially grown layer) is formed. In the substrate 10, an engraved
region 14 is so formed as to include the defect-concentrated region 11.
The engraved region 14 is engraved by RIE. Moreover, on the top of the
nitride semiconductor layer 13, a ridge portion 15 that serves as a laser
light waveguide structure and a SiO2 layer 16 for current
constriction are formed, and on top of this, a p-type electrode 17 is
formed. Furthermore, on the bottom face, of the substrate 10, an n-type
electrode 18 is formed.

[0048]Note that, in the present specification, the distance from the
center of the ridge portion 15 to an edge of the engraved region 14 is
represented by d. In FIG. 1(a), it is assumed that d=40 μm.

[0049]FIG. 2 is a sectional view showing the layer structure of the
nitride semiconductor layer 13. The nitride semiconductor layer 13 has
the following layers formed one on top of another in the order mentioned
on the surface of an n-type GaN layer 20 (with a film thickness of 3.5
μm): an n-type Al0.062Ga0.938N first clad layer 21 (with a
film thickness of 2.3 μm), an n-type Al0.1Ga0.9N second clad
layer 22 (with a film thickness of 0.2 μm), an n-type
Al0.062Ga0.938N third clad layer 23 (with a film thickness of
0.1 μm), an n-type GaN guide layer 24 (with a film thickness of 0.1
μm), an InGaN/GaN-3MQW active layer 25 (with an InGaN/GaN film
thickness of 4 nm/8 nm), a p-type Al0.3Ga0.7N vaporization
prevention layer 26 (with a film thickness of 20 nm), a p-type GaN guide
layer 27 (with a film thickness of 0.05 μm), a p-type
Al0.062Ga0.938N clad layer 28 (with a film thickness of 0.5
μm), and a p-type GaN contact layer 29 (with a film thickness of 0.1
μm).

[0050]As shown in FIG. 1(b), the line-shaped defect-concentrated region 11
extends in the direction [1-100]. The defects, which are linear as seen
from above, may have different shapes depending on their defect density
and type. Examples of the shape of the defect-concentrated region are
shown in FIGS. 3(a) to 3(c). There are, for example, defect-concentrated
regions in the shape of lines (FIG. 3(a)), defect-concentrated regions in
the shape of holes, (FIG. 3(b)), and closely-spaced defect-concentrated
regions in the shape of fine holes (FIG. 3(c)). The size of the holes and
linear cores here is of the order of about 1 nm to several tens of μm.
This embodiment deals with a case shown in FIG. 3(a). Note that the same
advantages are obtained in cases shown in FIGS. 3(b) and 3(c).

[0051]Next, a fabricating procedure will be described. As in the
conventional example described earlier, the GaN substrate 10 having
defect-concentrated regions in the shape of lines is fabricated through
the following procedure. On a sapphire substrate, a 2.5 μm thick
primer GaN layer is grown by MOCVD. Then, on top of this, a SiO2
mask pattern having regular stripe-shaped openings is formed (with a
period of 20 μm), and then a 15 μm thick GaN layer is formed again
by MOCVD to produce a wafer. The film does not grow on SiO2, and
thus starts to grow inside the openings. As soon as the film becomes
thicker than the SiO2, the film then starts to grow horizontally
away from the openings. At the center of every SiO2 segment,
different portions of the film growing from opposite sides meet,
producing, where they meet, a defect-concentrated region 11 with high
defect density. Since the SiO2 is formed in the shape of lines,
defect-concentrated regions are also formed in the shape of lines. Here,
the width of the defect-concentrated region 11 is about 40 μm, and the
defect-concentrated regions 11 are formed at about 400 μm intervals.

[0052]Here, the substrate is produced by ELOG. It should be understood,
however, that other fabricating methods may be used. Specifically, the
only requirement is to use a substrate including a defect-concentrated
region and a low-defect region and grow a nitride semiconductor layer on
the substrate. The substrate may be a substrate of sapphire, or a
substrate of another material, for example, a substrate of SiC, GaN,
GaAs, Si, spinel, or ZnO.

[0053]Next, all over the surface of the substrate 10, SiO2 or the
like is vapor-deposited by electron beam deposition so as to have a
thickness of 400 nm. Then, by common photolithography, stripe-shaped
windows are formed with photoresist in the direction [1-100] so as to
have a width of 60 μm each and include a defect-concentrated region
each. Then, by ICP or RIE, the SiO2 and the GaN substrate 10 are
etched. The GaN substrate 10 is etched to a depth of 4 μm. Thereafter,
the SiO2 is removed with an etchant such as HF. This is the end of
the treatment of the substrate to be performed before a nitride
semiconductor layer 13 is grown thereon.

[0054]FIG. 4 shows the thus obtained substrate 10. FIG. 4(a) is a top view
of the substrate 10, and FIG. 4(b) is a sectional view of FIG. 4(a).
Reference numeral 14a represents the regions etched by RIE so as to
include the defect-concentrated region 11. Symbol X represents the
etching depth. In the present specification, the etching may be achieved
by the use of gas-phase etching, or by the use of a liquid etchant.

[0055]Then, the nitride semiconductor layer 13 is laid on the top of the
substrate 10, followed by the formation of the ridge portion 15, the
SiO2 layer 16, the p-electrode 17, and the n-electrode 18.

[0056]When the defect-concentrated region 11 in the substrate 10 is
engraved by RIE, and the nitride semiconductor layer 13 is then laid on
the top of the substrate 10, the surface flatness of the engraved region
14 is greatly degraded to the same level of roughness as that of the
conventional nitride semiconductor laser device shown in FIG. 16 (see
FIG. 18). However, as shown in FIG. 5(b), the level difference between
the highest and lowest parts, within the 600 μm wide region, except
the engraved region 14, in which the measurement was taken, was found to
be 20 nm or less. Here, the drops corresponding to the groove portions
shown in FIG. 5(a) are not considered.

[0057]The reasons are explained by using FIG. 6. FIG. 6(a) is a top view
showing how the nitride semiconductor layer 13 having the
defect-concentrated regions 11 in the shape of lines grows, and FIG. 6(b)
is a top view showing how the nitride semiconductor layer 13 having the
defect-concentrated regions 11 in the shape of dots grows. Unlike the
case in FIG. 17, where the growth direction varies depending on the shape
of the defect-concentrated region 11, the formation of the engraved
region 14 makes it possible to achieve approximately the same growth
direction as indicated by arrows C and D shown in FIG. 6, preventing the
growth meet portion from being produced due to the difference in the
growth direction. This prevents the thickness of the individual layers
from being varied within the surface, making uniform the layer thickness
thereof.

[0058]Moreover, as shown in FIGS. 6(a) and 6(b), the engraved region 14
makes it possible to achieve the same growth direction within the surface
regardless of the shape of the defect-concentrated region 11, and is thus
effective in improving the surface flatness.

[0059]By forming the ridge portion 15 on the thus obtained extremely flat
region, it is possible to suppress the in-surface distribution of the
device characteristics and thus improve the yield rate dramatically. The
useful lives of the thus obtained semiconductor laser devices were tested
with the devices driven under APC at 60° C. and at an output of 30
mW. Here, the useful life is defined as the length of time required for
Iop (a current value when the optical output is kept at 30 mW) to
become 1.5 times the initial level thereof. In the test, the devices
emitted at wavelengths of 405±5 nm. From each wafer, 50 semiconductor
laser devices were randomly picked out, and the number of devices of
which the useful lives exceeded 3,000 hours was counted as the yield
rate.

[0060]Here, the yield rate was more than 80%. Note that, when the nitride
semiconductor layer 13 was grown directly on the substrate 10 shown in
the conventional example described earlier, the yield rate was 30% or
less. Accordingly, it can be said that better surface flatness of the
nitride semiconductor layer 13 (except the engraved region 14) makes
uniform the layer thickness and the composition of the individual layers
within the wafer surface, leading to better yield rate.

[0061]Now, the depth X of the engraved region 14 shown in FIG. 4 will be
explained. FIG. 7 shows the relationship between the engraving depth X
and the yield rate. Although FIG. 7 shows an example in which the deepest
engraving depth X is 5 μm, the yield rate was found to be more than
80% even when the depth was more than 5 μm. If the engraving depth X
is less than 0.5 μm, the engraved region is filled quickly when the
primer n-type GaN grows. Thus, the poor surface flatness of the engraved
region 14 spreads out of it to degrade the surface flatness of the region
outside the engraved region 14. Moreover, it has been found that, if X=50
μm or more, when, in general, in the device separation process, the
substrate is polished and ground, cracks or the like develop, resulting
in a low yield rate. Hence, it is preferable that the engraving depth X
be 0.5 μm or more but 50 μm or less.

[0062]Now, the position of the engraved region 14 will be explained. FIG.
8(a) is a top view of the substrate 10 having the defect-concentrated
regions in the shape of lines, and FIG. 8(b) is a top view of the
substrate 10 having the defect-concentrated regions in the shape of dots.
As shown in FIGS. 8(a) and 8(b), the distance from an edge of the
defect-concentrated region 11 to an edge of the engraved region 14 is
represented by Y. Here, although the distance Y on one side of the
defect-concentrated region 11 in the width direction differs from the
distance Y on the other side of the defect-concentrated region 11, a
shorter one is defined as the distance Y.

[0063]FIG. 9 shows the relationship between the distance Y and the yield
rate. If the distance Y is less than 5 μm, the engraved region 14
cannot accommodate all the low-crystallinity portions of the
defect-concentrated region 11, letting them be outside the engraved
region 14, resulting in a low yield rate. Hence, it is preferable that
the distance Y be 5 μm or more.

[0064]Now, the position of the ridge portion 15 will be explained. The
position of the ridge portion 15 is defined by the distance d shown in
FIG. 1. If the distance d is less than 5 μm, there appear edge growths
(i.e., the growth rate of the edge portions of the unengraved region
increases, making the layer thicker), resulting in the variation in the
layer thickness. This is undesirable. Hence, no problem arises when the
distance d is 5 μm or more.

[0065]In the nitride semiconductor laser device shown in the conventional
example described earlier, the number of cracks observed per 1 cm2
area on the nitride semiconductor laser device 13 is five to seven. The
reason is believed to be strains produced by the differences in lattice
constant or in the thermal expansion coefficient between the AlGaN clad
layer and the GaN layer included in the nitride semiconductor layer 13.
Such cracks present in a chip greatly affect the characteristics of a
nitride semiconductor device, resulting in a low yield rate.

[0066]By contrast, in the nitride semiconductor laser device of this
embodiment, the number of cracks observed per 1 cm2 area is zero.
Thus, with this embodiment, it is possible to greatly reduce the number
of cracks in the nitride semiconductor layer 13. The reason is believed
to be that the strains present within the nitride semiconductor layer 13
are released by the presence of the engraved region 14.

Second Embodiment

[0067]This embodiment deals with a case where the defect-concentrated
region 11 has the shape of a dot. This embodiment has the same process
and configuration, etc. as those of the first embodiment except in that,
here, the defect-concentrated region 11 in the substrate 10 has the shape
of a dot.

[0068]When no engraved region 14 was formed as in the conventional example
described earlier, the nitride semiconductor layer 13 grew concentrically
away from the defect-concentrated region 11, and the flatness in a growth
meet portion was greatly degraded. We then measured the surface
roughness, and observed that the level difference between the highest and
lowest parts on the surface was as great as 200 nm.

[0069]FIG. 10 is a top view of the substrate 10 having the
defect-concentrated region 11 in the shape of a dot. By forming the
engraved region 14 in the shape of a line so as to include the dot-shaped
defect-concentrated regions 11, it is possible to improve the surface
flatness.

[0070]We measured the surface flatness of the wafer produced in the manner
described in the first embodiment, and observed that the level difference
between the highest and lowest parts on the surface was 20 nm or less.
Moreover, the obtained yield rate was approximately the same as that of
the first embodiment. Furthermore, it is preferable that the depth X of
the engraved region, the distance Y, and the distance d be made equal to
those in the first embodiment.

Third Embodiment

[0071]In this embodiment, a substrate having a depression is used. This
depression may be formed elsewhere than in the defect-concentrated region
11. This embodiment has the same process and configuration, etc. as those
of the first embodiment except for the substrate to be used.

[0072]The depression can take different shapes. Examples of the shape of
the depression are shown in FIG. 11. FIG. 11(a) is a top view of the
substrate, and FIG. 11(b) is a sectional view of FIG. 11(a). Here,
assuming that these depressions 30a to 30c have the width Z and the depth
V. Experiments have proved that, if the width Z is 1 μm or more and
the depth V is 0.5 μm or more, the growth of the nitride semiconductor
layer 13 occurs in different directions according to the shape of the
depression. On the other hand, although the depression smaller and
shallower than that described above is filled quickly and thus does not
affect the growth direction, it degrades the surface flatness.

[0073]The region having such a depression is engraved, as in the first
embodiment, by the use of gas-phase etching such as RIE. Then, a wafer is
produced in the same manner as in the first embodiment. We then measured
the surface flatness, and observed that the level difference between the
highest and lowest parts on the surface was 20 nm or less. On the other
hand, when the nitride semiconductor layer 13 is grown without forming
the engraved region 14 as in the conventional example described earlier,
the level difference between the highest and lowest parts on the surface
was greatly degraded to 200 nm or more.

[0074]Moreover, the obtained yield rate was approximately the same as that
of the first embodiment. Furthermore, it is preferable that the depth X
of the engraved region, the distance Y, and the distance d be made equal
to those in the first embodiment.

Fourth Embodiment

[0075]In this embodiment, a semiconductor laser device having an engraved
region other than the engraved region 14 including the
defect-concentrated region 11 will be explained.

[0076]This embodiment has the same process and configuration, etc. as
those of the first embodiment except for the position of the engraved
region on the substrate 10.

[0077]FIG. 12(a) is a top view of the substrate of the fourth embodiment,
and FIG. 12(b) is a sectional view of FIG. 12(a). There exist, as the
engraved region, the engraved region 14 including the defect-concentrated
region 11 and an engraved region 14a formed in the low-defect region 12.

[0078]The engraved region 14a is provided for the purpose of preventing,
when an abnormal growth portion such as a region where defects or growth
surfaces are different than elsewhere is included in the low-defect
region, such a portion from affecting a widespread area. FIG. 13(a) is a
top view showing how the nitride semiconductor layer 13 having no
engraved region 14a grows, and FIG. 6(b) is a top view showing how the
nitride semiconductor layer 13 having the engraved region 14a grows. As
shown in FIG. 16(a), when there exists an irregular defect or the like in
the low-defect region 12, abnormal growth occurs there, because there is
no engraved region, and then spreads over the low-defect region 12.
However, it has been found that, by forming the engraved region 14a also
in the low-defect region 12, it is possible to prevent the abnormal
growth from spreading out of it as shown in FIG. 16(b). Specifically, the
engraved region 14a prevents the abnormal growth occurred in a low-defect
region 12a of FIG. 16(b) from spreading out of it, allowing the
low-defect region 12b to maintain better surface flatness.

[0079]It has been found that the width of the engraved region 14a should
be 3 μm or more to prevent the abnormal growth occurred in the
low-defect region 12a from spreading out of it into the low-defect region
12b and keep the level difference of the surface flatness to be 20 nm or
less. If the width was 3 μm or less, the engraved region 14a was
filled, making it impossible to prevent abnormal growth. If the width is
200 μm or more, however, the area of the low-defect region 12 is
reduced. This makes narrower the region on which the p-electrode 17 or
the like is to be formed, resulting in low process yield. This is
undesirable.

[0080]For the same reason as stated in the first embodiment, it is
preferable that the engraving depth X of the engraved region 14a be 0.5
μm or more but 50 μm or less.

[0081]Note that a plurality of engraved regions 14a may be provided
between the engraved regions 14, and the same advantage can be achieved
by forming them anywhere within the low-defect region.

Fifth Embodiment

[0082]This embodiment deals with a case where, instead of engraving the
defect-concentrated region 11 by the use of etching such as RIE, engraved
regions are formed on both sides of the defect-concentrated region 11 to
improve the surface flatness of the nitride semiconductor layer 13,
achieving greatly improved in-surface yield rate of the characteristics
of a semiconductor laser device. This embodiment has the same process and
configuration, etc. as those of the first embodiment except for the
position of the engraved region on the substrate 10.

[0083]FIG. 14(a) is a top view of the substrate of the fifth embodiment,
and FIG. 14(b) is a sectional view of FIG. 14(a). Here, engraved regions
14b are provided on both sides of the defect-concentrated region 11. For
example, the width of the engraved region 14b can be set to 20 μm and
the depth thereof at 3 μm. Reference numeral 175 represents a
low-defect region between the engraved regions 14b, and is referred to as
a ridge portion formation region. The ridge portion formation region is a
region where a ridge portion that serves as a light waveguide region
formed on the top of the nitride semiconductor layer 13 grown on the
substrate 10 for the purpose of producing a nitride semiconductor laser
device.

[0084]On the substrate 10 of FIG. 14, the nitride semiconductor layer 13
is epitaxially grown. Then, a nitride semiconductor laser device is
fabricated on the thus obtained wafer. FIG. 15(a) is a sectional view of
the nitride semiconductor laser device of the fifth embodiment, and FIG.
15(b) is a top view of FIG. 15(a). Also here, just as in FIG. 1(a), the
distance from the center of the ridge portion 15 to an edge of the
engraved region 14b is represented by d, and d=100 μm.

[0085]When, as in this embodiment, the engraved regions 14b are provided
on both sides of the defect-concentrated region 11 in the substrate 10,
and the nitride semiconductor layer 13 is grown on the substrate 10, the
surface flatness of a region that includes the defect-concentrated region
11 and is sandwiched between the engraved regions 14b is greatly
degraded.

[0086]However, even after the nitride semiconductor layer 13 was
epitaxially grown, the level difference between the highest and lowest
parts, within the 600 μm wide region in which the surface flatness of
the ridge portion formation region 12c shown in FIG. 14 was measured, was
found to be 20 nm or less. The reason is believed to be that, with action
similar to the engraved region 14a of FIG. 13, an abnormal growth region
can be prevented from spreading. Thus, it has been found that it is
possible to improve the surface flatness by forming the engraved region
14b in the low-defect region 12 so as to include the defect-concentrated
region 11 without having to engrave the defect-concentrated region 11.
With this engraved region, it is possible to suppress the in-surface
distribution of the device characteristics and thus improve the yield
rate dramatically.

[0087]Moreover, the obtained yield rate was approximately the same as that
of the first embodiment. Furthermore, it is preferable that the depth X
of the engraved region, the distance Y, and the distance d be made equal
to those in the first embodiment.

[0088]Moreover, to keep the level difference of the surface flatness to be
20 nm or less, it is preferable that the width of the engraved region 14b
be 3 μm or more but 150 μm or less. If the width is 3 μm or
less, the engraved region 14b is filled, making the abnormal growth in
the defect-concentrated region 11 spread into the low-defect region 12.
If the width is 150 μm or more, however, the area of the low-defect
region 12 is reduced. This makes narrower the region on which the
p-electrode 17 or the like is to be formed, resulting in low process
yield. This is undesirable.

[0089]Furthermore, in the nitride semiconductor laser device of this
embodiment, the number of cracks observed per 1 cm2 area is zero.
Thus, with this embodiment, it is possible to greatly reduce the number
of cracks in the nitride semiconductor layer 13 for the same reason as
stated in the first embodiment.

INDUSTRIAL APPLICABILITY

[0090]A nitride semiconductor light-emitting device according to the
present invention can be used effectively, especially in a nitride
semiconductor laser device.